1. Field of the Invention
The present invention provides a continuous electrochemical process for the preparation of zinc powder from zinc oxide.
2. Description of the Related Art
Zinc powder is widely used in the chemical industry in various industries. Zinc oxide containing other zinc salts, metal impurities, etc. is produced as a byproduct. Recycling of the zinc oxide to produce pure zinc powder is highly desirable from a cost as well as an environmental point of view.
The electrodeposition of zinc metal is a well-known reaction in electrochemical technology (See, for example, D. Pletcher and F. C. Walsh, Industrial Electrochemistry, Blackie Academic, 1993). The electrogalvanizing of steel is a process carried out on a very large scale and aqueous acid is the normal medium. High speed, reel to reel galvanizing of steel is carried out in sulfuric acid with dimensionally stable anodes and uniform deposition is achieved at high current density by inducing very efficient mass transport by rapid movement of the steel surface. The deposition of zinc metal is also the critical electrode reaction in the electrowinning and electrorefining of zinc. In addition, there are a number of technologies, which have been demonstrated for the removal of Zn(II) from effluents. However, in these technologies, concentration of Zn(II) is low, commonly less than 100 ppm. Finally, the deposition of zinc has been widely investigated as the cathodic reaction in candidate secondary batteries. In all these applications, however, the objective is to select the conditions so as to give an adhesive and smooth zinc coating.
Zinc powder can be produced by electrolysis either in strong alkaline or neutral zinc containing solutions. The first patents obtained on the alkaline electrolysis process date back to the early thirties (German Patents, 581013, 506590, 653557). In these methods, concentration of zinc was low (approximately 30 grams per liter) and a low current density of 1200-1500 amperes/sq. meter was used. Volume efficiency and current density of these batch type processes are too low to be industrially attractive. I. Orszagh and B. Vass (Hung. J. Ind. Chem., 13,(1985) 287) used these methods to recycle zinc oxide byproduct from zinc dithionite production. They, however, used a divided cell at a low current density (1000-3000 A/m2). Use of a divided cell and low current density makes this process significantly more capital intensive. In their study, no significant difference was observed at different sodium hydroxide concentrations.
For the recycling of zinc oxide containing waste by an alkaline electrolysis process to be industrially attractive, the alkaline electrolysis process needs to be improved to lower capital as well as operational expenses. Capital expenses can be significantly reduced by increasing the current density and by providing a process that is capable of being carried out in an undivided cell. Furthermore, electrolysis conditions need to be improved to achieve high volume efficiency and minimum corrosion of the electrodes. The present invention unexpectedly fulfills these and other needs.
One advantage of the present invention is that by providing a continuous process for the electrochemical reduction of zinc oxide (or any other zinc compound that reacts with an aqueous base to produce zinc oxide) to zinc powder, it provides for a very high volume efficiency, as solid zinc oxide (or the other zinc compound) is added continuously during the electrochemical process to maintain the concentration of zinc based species (such as Zn2+ ions). Furthermore, the use of high current density coupled with high volume efficiency makes the presently claimed process industrially attractive.
J. St-Pierre, D. L. Piron (Electrowinning of zinc from alkaline solutions at high current densities; J. Appl. Electrochem (1990), 20(1), 163-5), discloses experimental results conducted at a current density of about 2000 to 8000 A/m2 to obtain cell voltage and current efficiency data necessary for specific energy computations.
U.S. Pat. No. 5,958,210 discloses a method for electrowinning metallic zinc from zinc ion in aqueous solution, said method comprising performing electrolysis on a mixture of solid conductive particles and aqueous alkali solution, said solution ranging in concentration from about 3N to about 20N alkali and containing dissolved zinc ion at an initial concentration ranging from about 50 to about 500 grams of zinc ion per liter of said solution, in an electrolytic cell containing first and second vertically arranged, parallel flat plates defined as a current feeder and a counter electrode, respectively, said counter electrode coated with a substance that is catalytic for oxygen evolution, said cell further containing an ion-permeable diaphragm parallel to each of said plates and interposed therebetween to define a gap between said current feeder and said diaphragm, by passing said mixture of particles and solution through said gap such that said particles contact said current feeder and passing a current across said gap, thereby depositing metallic zinc from said solution onto said particles. The electrowinning process is disclosed to yield high current efficiency and low energy consumption. The process, however, is not industrially attractive for a large scale production of zinc powder because this process uses a relatively more complex cell, and a lower current density.
The present invention provides a continuous electrochemical process for preparing zinc powder, which comprises the steps of:
a) providing to an electrochemical cell a solution or suspension in an aqueous 1.25 Molar to 10.0 Molar base solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the solution or suspension containing at least 2 millimoles of solubilized zinc based species per 100 grams of electrolyte; and
b) passing current to the cell at a current density of about 500 to 40,000 A/m2, for a time period sufficient to electrochemically reduce the zinc based species to zinc powder, while continuously or intermittently adding a sufficient amount of the zinc oxide or the other zinc compound to the cell to maintain the concentration of the zinc based species at a level of at least 2 millimoles per 100 grams of electrolyte and continuously or intermittently removing at least a portion of the zinc powder formed;
wherein in steps a) and b), the electrolyte consists of the aqueous base solution and the zinc oxide or the other zinc compound, and the solubilized zinc species are derived from the zinc oxide or the other zinc compound.
The first step a) of the presently claimed electrochemical process for preparing zinc powder involves: providing to an electrochemical cell a solution or suspension in an aqueous 1.25 Molar to 10.0 Molar base solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the solution or suspension containing at least 2 millimoles of solubilized zinc based species per 100 grams of electrolyte.
As used herein, the phrase xe2x80x9czinc powderxe2x80x9d encompasses zinc metal particles of various particle sizes known to one of ordinary skill in the art and is not limited to fine particles.
The electrolytic cell employed in the present invention may be an undivided or divided cell, with the undivided cell being preferred. Use of an undivided cell requires lower capital. Furthermore, operational costs are also lower when an undivided cell is used. Therefore, it is important that the process be capable of being carried out in an undivided cell, if desired.
Since zinc powder formed at the cathode by the reduction reaction can react with the oxygen generated at the anode, cathodic and anodic chemistries are generally separated by some kind of a porous diaphragm which allows the current to pass, but suppresses mixing of anolyte and catholyte. Cells of this kind are called divided cells.
The design of the undivided cell is simpler and the cell voltage required to achieve the desired current density is lower because of the lower ohmic resistance. This means that the electrical cost is generally lower where an undivided cell is used. Furthermore, capital cost required with the undivided cell is significantly lower than the divided cell.
The anode may be made from any conventional suitable material such as platinum, or iridium, either of which may be coated over an inert support such as niobium or titanium. The anode may also be made of nickel, or from conventional materials having good alkali corrosion resistance, e.g., lead or stainless steel. The cathode may be made from any conventional suitable materials having good alkali corrosion resistance, such as magnesium nickel, lead and stainless steel. Preferably, the anode in the present invention is formed of stainless steel and the cathode is formed of stainless steel, magnesium, or magnesium alloy.
The solubilized zinc based species are obtained by dissolving zinc oxide, or the other zinc compound (such as zinc sulfate) in an aqueous 1.25 M to 10.0 M base solution, and in one embodiment 2.5 M to 5.0 M, and in one embodiment 5.0 to 10.0 M base solution. High concentration (preferably obtained from a saturated solution of, or a light slurry of zinc oxide) of the solubilized zinc based species in the electrolyte is maintained during the electrolysis by adding ZnO or the other zinc compound either continuously or intermittently during the electrolysis.
Examples of solubilized zinc based species include ZnO22xe2x88x92, HZnO21xe2x88x92, Zn(OH)+, and Zn2+. Zinc oxide is known to dissolve by reacting with water to form a variety of species (which includes ionic and neutral species) depending upon pH. Thus a solution of zinc oxide in alkaline solution may contain species such as ZnO22xe2x88x92, HZnO21xe2x88x92, Zn(OH)2, Zn(OH)+, and Zn2+. Therefore, solubilized zinc based species may comprise one or more of these species in the solution.
The concentration of solubilized zinc based species provided to the electrochemical cell is at least 2 millimoles (mmoles) per 100 grams (g) of electrolyte. The electrolyte comprises the aqueous 1.25 Molar to 10.0 Molar base solution and the zinc oxide or the other zinc compound. Preferably, the concentration of the solubilized zinc based species is at least 20 mmoles per 100 grams of electrolyte, and in one embodiment at least 30 mmoles per 100 grams of the electrolyte, and in one embodiment ranges from 2 to 120 mmoles per 100 grams of the electrolyte. By way of example, 2 weight percent of zinc oxide in the electrolyte corresponds approximately to 30 mmoles of solubilized zinc based species per 100 grams of the electrolyte. Also by way of example, 2 millimoles of zinc based species per 100 grams of electrolyte could be provided by dissolving 0.16 grams (0.002xc3x9781.37=0.16) of zinc oxide in 99.84 grams of the 1.25 M to 10.0 M aqueous base solution.
Concentration of the aqueous base solution (such as caustic solution) should be as high as possible because of the higher solubility of zinc oxide in more concentrated aqueous base solutions. In order to achieve high current density (which is a function of the concentration of the electroactive species such as zinc based species), high current efficiency, and high volume efficiency (i.e., high amount of zinc produced per volume unit of electrolyte) it is desirable to use the electrolytes containing high concentration of solubilized zinc based species. Because of the higher solubility of zinc oxide in stronger base solutions, higher concentration of solubilized zinc based species is achieved by using stronger base solutions. By maintaining a light suspension of zinc oxide during the electrolysis in the present invention, the concentrations of solubilized zinc based species during the electrochemical process can be kept as high as possible. However, the concentration of solubilized zinc based species can be increased by using a stronger initial base solution used for solubilizing zinc oxide. It is generally believed that a higher concentration of base solution is favorable for minimizing corrosion of the anode. However, it has been unexpectedly found that high concentration (such as concentration significantly higher than 10.0 M) of base (e.g., NaOH solution) solution causes an adverse effect on corrosion of the stainless steel anode and on the current efficiency of the process especially where electrolysis is carried out at high current densities (such as higher than 10,000 A/m2). This adverse effect is minimized by lowering the aqueous base concentration. The most preferred concentration of the aqueous base in the present invention is 3.0 to 5.0 M.
The aqueous base solutions employed in the process of the invention are prepared by combining water with a source of alkali metal or alkaline earth metal ions, such as lithium sodium and potassium, and a source of hydroxyl (OHxe2x88x92 ions). A single source may of course provide both types of ions. The various alkali or alkaline earth metal ions are preferably supplied from various compounds such as hydroxides and oxides. Preferred base solutions are sodium and potassium hydroxide solutions.
The solubility of zinc oxide in the aqueous base solution is limited, and depends on the temperature. The present invention envisions use of the zinc oxide at any range of concentrations in which it is soluble in the aqueous base solution. However, the concentration of the electroactive speciesxe2x80x94in this case solubilized zinc based speciesxe2x80x94is a major variable that determines the maximum feasible current density at which zinc is electrodeposited at maximum current efficiency. An excessive current density will generally lead to secondary reactions such as hydrogen evolution (with a potential for some safety problems). Thus, too low a concentration of zinc based species in the electrolyte at high current densities will lead to lower current efficiency and hence is undesirable from cost and safety considerations. Hence, in the present invention, a saturated solution or a light suspension of zinc oxide (or the other zinc compound that produces zinc oxide upon reaction with aqueous base) is maintained. In one embodiment, the zinc oxide or the other zinc compound which produces the zinc oxide is present in the aqueous base in an amount of 0.15 to 12 weight percent (wt.%) calculated at 90xc2x0 C., and in one embodiment from 1.5 to 6 wt. % calculated at 90xc2x0 C., based on the weight of the electrolyte. At 90xc2x0 C., a saturated solution of zinc oxide in 4.0 M NaOH contains 2 wt % of zinc oxide, based on the total weight of the electrolyte.
The second step b) of the presently claimed invention involves passing current to the cell at a current density of 500 to 40,000 amps per square meter A/m2, preferably 1,000 to 40,000 A/m2, and in one embodiment from 1000 to 5,000, in one embodiment, 10,000 to 20,000, in one embodiment 20,000 to 30,000, and in one embodiment, 30,000 to 40,000 A/m2, and in one embodiment 10,000 to 40,000 A/m2, for a time period sufficient to electrochemically reduce the solubilized zinc based species to zinc powder. Since the present process is a continuous process, while current is passed to the cell, a sufficient amount of zinc oxide or the other zinc compound is added to the cell continuously or intermittently to maintain the concentration of the solubilized zinc based species at a level of at least 2 mmoles of solubilized zinc based species per 100 grams of the electrolyte. Also, while current is passed to the cell, at least a portion of the zinc powder formed is removed continuously or intermittently.
While the present invention is not limited in scope by current efficiency considerations, in one embodiment, the present electrochemical process has a current efficiency of at least 70% (i.e., current efficiency of 70-100%), and in one embodiment at least 80%, and in one embodiment at least 90%. As used herein, the phrase xe2x80x9ccurrent efficiencyxe2x80x9d is the ratio, generally expressed as a percentage, of the actual zinc deposition rate to the rate which would be achieved if all of the current passing through the cell were consumed by reduction of zinc ion. The current efficiencies in zinc electrowinning cells are typically less than 100% because of the concurrent reduction of water to hydrogen gas, competing with the reduction of the solubilized zinc based species (e.g., from zinc oxide) to zinc metal at the cathode.
For electrolysis, temperatures higher than ambient are generally desired because of the beneficial effects on the kinetics of all steps in an electrode process. At higher temperatures, the diffusion coefficient, the exchange current density and the rates of chemical reactions generally are increased. The decrease in viscosity and increase in diffusion coefficient leads to the increased mass transport rates. This increased mass transport of zinc species from the bulk of the solution to the cathodic region is highly desirable. However, increase in the rate of chemical reaction such as the oxidation of zinc produced with oxygen and mass transport of the byproduct oxygen to the bulk of the solution may not be desirable. In the present invention, higher than ambient temperatures are found to be favorable for the electrolytic reduction of zinc oxide to zinc, and are thus preferred.
Thus, in one embodiment, the presently claimed electrochemical reduction process is conducted at a temperature of from 30xc2x0 C. to 120xc2x0 C., preferably from 50xc2x0 C. to 110xc2x0 C., and more preferably from 70xc2x0 C. to 100xc2x0 C.
In one embodiment of the present invention, the electrochemical process is substantially free of electrode corrosion. As used herein, the phrase xe2x80x9csubstantially free of electrode corrosionxe2x80x9d encompasses corrosion levels, expressed as milligrams of electrode metal lost to corrosion/mole of electrons passed of 100 or less (i.e. xe2x89xa6100 mg/mole of electrons). In one embodiment, the corrosion levels are less than or equal to 50 mg/mole of electrons, and in one embodiment less than or equal to 40, 30, 20, 10, and 5 mg/mole of electrons. Methods for measuring corrosion levels will be known to one of ordinary skill in the art. In one embodiment, as in the present invention, it is measured by analysis of the recovered zinc powder for iron ion concentration by atomic absorption spectroscopy or inductively coupled plasma, as iron lost by corrosion is insoluble in the aqueous base solutions of the present invention.
The following specific examples will provide detailed illustrations of the methods of producing and utilizing compositions of the present invention. These examples are not intended, however, to limit or restrict the scope of the invention in any way and should not be construed as providing conditions, parameters or values which must be utilized exclusively in order to practice the present invention. Unless otherwise specified, all parts and percents are by weight.